A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In addition to Extreme Ultra Violet (EUV) radiation, radiation sources used in EUV lithography tend to generate contaminant material that may be harmful for the optics and the working environment in which the lithographic process is carried out. This is especially the case for EUV sources that operate via a discharge-produced plasma source, such as a plasma tin source. Such a source typically comprises a pair of electrodes to which a voltage can be applied. In addition, a vapor is produced, for example, by a laser beam that is targeted to, for example, one of the electrodes. Accordingly, a discharge will occur between the electrodes, thereby generating a plasma, which causes a so-called pinch in which EUV radiation is produced. In addition to this radiation, the discharge source typically produces debris particles varying in size from atomic to larger than a micrometer, which can be both charged and uncharged. It is desired to limit the contamination of the optical system that is arranged to condition the beams of radiation coming from an EUV source by the debris. Conventional shielding of the optical system primarily includes a system comprising a high number of closely packed foils aligned parallel to the direction of the light generated by the EUV source. A so-called foil trap, for instance, as disclosed in EP1491963 uses a high number of closely packed foils aligned generally parallel to the direction of the light generated by the EUV source. Contaminant debris, such as micro-particles, nano-particles and ions can be trapped in walls provided by the foil plates. Thus, the foil trap functions as a contamination barrier that traps contaminant material from the source. Due to the arrangement of the platelets the foil trap is transparent for light, but will capture debris, either because the debris is not traveling parallel to the platelets, or because of a randomized motion caused by a buffer gas. For contamination barriers in an EUV apparatus that are exposed to contamination from a tin debris generating radiation source over time, a desire exists to provide a cleaning method, since the barriers will be contaminated with captured debris, which may impede the radiation transmissive properties of the barrier.